Meaningful error message if REST API server is not reachable

app/controllers/login.js

@@ -2,6 +2,7 @@ import Controller from '@ember/controller';
 import { service } from '@ember/service';
 import { action } from '@ember/object';
 import { tracked } from '@glimmer/tracking';
+import { base_url } from 'papermerge/utils';
 
 
 export default class LoginController extends Controller {
@@ -18,7 +19,14 @@ export default class LoginController extends Controller {
         password
       );
     } catch (error) {
-      this.errorMessage = "Invalid credentials";
+      if (error instanceof TypeError) {
+        // TypeError is thrown by fetch API if there is a
+        // network issue e.g. backend server is not reachable;
+        let extra_message = `Please double check that REST API server '${base_url()}/' is reachable.`
+        this.errorMessage = `${error} ${extra_message}`;
+      } else {
+        this.errorMessage = error;
+      }
     }
 
     if (this.session.isAuthenticated) {